From 3985a19c75fb5050a0a50c53a33635f2776d686f Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Thu, 22 Jun 2023 16:49:43 -0700
Subject: [PATCH] REF: separate out cross-merge, make less stateful

---
 pandas/core/reshape/merge.py | 140 +++++++++++++++++++----------------
 1 file changed, 75 insertions(+), 65 deletions(-)

diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index 4d1f8bd6301d0..1d857b6c4509e 100644
--- a/pandas/core/reshape/merge.py
+++ b/pandas/core/reshape/merge.py
@@ -145,10 +145,79 @@ def merge(
     indicator: str | bool = False,
     validate: str | None = None,
 ) -> DataFrame:
+    if how == "cross":
+        return _cross_merge(
+            left,
+            right,
+            on=on,
+            left_on=left_on,
+            right_on=right_on,
+            left_index=left_index,
+            right_index=right_index,
+            sort=sort,
+            suffixes=suffixes,
+            indicator=indicator,
+            validate=validate,
+            copy=copy,
+        )
+    else:
+        op = _MergeOperation(
+            left,
+            right,
+            how=how,
+            on=on,
+            left_on=left_on,
+            right_on=right_on,
+            left_index=left_index,
+            right_index=right_index,
+            sort=sort,
+            suffixes=suffixes,
+            indicator=indicator,
+            validate=validate,
+        )
+    return op.get_result(copy=copy)
+
+
+def _cross_merge(
+    left: DataFrame | Series,
+    right: DataFrame | Series,
+    on: IndexLabel | None = None,
+    left_on: IndexLabel | None = None,
+    right_on: IndexLabel | None = None,
+    left_index: bool = False,
+    right_index: bool = False,
+    sort: bool = False,
+    suffixes: Suffixes = ("_x", "_y"),
+    copy: bool | None = None,
+    indicator: str | bool = False,
+    validate: str | None = None,
+) -> DataFrame:
+    """
+    See merge.__doc__ with how='cross'
+    """
+
+    cross_col = f"_cross_{uuid.uuid4()}"
+    left = left.assign(**{cross_col: 1})
+    right = right.assign(**{cross_col: 1})
+
+    if (
+        left_index
+        or right_index
+        or right_on is not None
+        or left_on is not None
+        or on is not None
+    ):
+        raise MergeError(
+            "Can not pass on, right_on, left_on or set right_index=True or "
+            "left_index=True"
+        )
+
+    left_on = right_on = [cross_col]
+
     op = _MergeOperation(
         left,
         right,
-        how=how,
+        how="inner",
         on=on,
         left_on=left_on,
         right_on=right_on,
@@ -159,7 +228,9 @@ def merge(
         indicator=indicator,
         validate=validate,
     )
-    return op.get_result(copy=copy)
+    res = op.get_result(copy=copy)
+    del res[cross_col]
+    return res
 
 
 def _groupby_and_merge(by, left: DataFrame, right: DataFrame, merge_pieces):
@@ -706,17 +777,6 @@ def __init__(
 
         self.left_on, self.right_on = self._validate_left_right_on(left_on, right_on)
 
-        cross_col = None
-        if self.how == "cross":
-            (
-                self.left,
-                self.right,
-                self.how,
-                cross_col,
-            ) = self._create_cross_configuration(self.left, self.right)
-            self.left_on = self.right_on = [cross_col]
-        self._cross = cross_col
-
         # note this function has side effects
         (
             self.left_join_keys,
@@ -809,17 +869,8 @@ def get_result(self, copy: bool | None = True) -> DataFrame:
 
         self._maybe_restore_index_levels(result)
 
-        self._maybe_drop_cross_column(result, self._cross)
-
         return result.__finalize__(self, method="merge")
 
-    @final
-    def _maybe_drop_cross_column(
-        self, result: DataFrame, cross_col: str | None
-    ) -> None:
-        if cross_col is not None:
-            del result[cross_col]
-
     @final
     @cache_readonly
     def _indicator_name(self) -> str | None:
@@ -1437,53 +1488,12 @@ def _maybe_coerce_merge_keys(self) -> None:
                 self.right = self.right.copy()
                 self.right[name] = self.right[name].astype(typ)
 
-    @final
-    def _create_cross_configuration(
-        self, left: DataFrame, right: DataFrame
-    ) -> tuple[DataFrame, DataFrame, JoinHow, str]:
-        """
-        Creates the configuration to dispatch the cross operation to inner join,
-        e.g. adding a join column and resetting parameters. Join column is added
-        to a new object, no inplace modification
-
-        Parameters
-        ----------
-        left : DataFrame
-        right : DataFrame
-
-        Returns
-        -------
-            a tuple (left, right, how, cross_col) representing the adjusted
-            DataFrames with cross_col, the merge operation set to inner and the column
-            to join over.
-        """
-        cross_col = f"_cross_{uuid.uuid4()}"
-        how: JoinHow = "inner"
-        return (
-            left.assign(**{cross_col: 1}),
-            right.assign(**{cross_col: 1}),
-            how,
-            cross_col,
-        )
-
     def _validate_left_right_on(self, left_on, right_on):
         left_on = com.maybe_make_list(left_on)
         right_on = com.maybe_make_list(right_on)
 
-        if self.how == "cross":
-            if (
-                self.left_index
-                or self.right_index
-                or right_on is not None
-                or left_on is not None
-                or self.on is not None
-            ):
-                raise MergeError(
-                    "Can not pass on, right_on, left_on or set right_index=True or "
-                    "left_index=True"
-                )
         # Hm, any way to make this logic less complicated??
-        elif self.on is None and left_on is None and right_on is None:
+        if self.on is None and left_on is None and right_on is None:
             if self.left_index and self.right_index:
                 left_on, right_on = (), ()
             elif self.left_index:
@@ -1551,7 +1561,7 @@ def _validate_left_right_on(self, left_on, right_on):
                         'of levels in the index of "left"'
                     )
                 left_on = [None] * n
-        if self.how != "cross" and len(right_on) != len(left_on):
+        if len(right_on) != len(left_on):
             raise ValueError("len(right_on) must equal len(left_on)")
 
         return left_on, right_on